With the development and increasing popularity of a graphical user interface (GUI) system, touch panels with simple input structure have been widely used. The touch panels include a resistive film type, a capacitive type, an optical sensor type, an ultrasonic type, an electromagnetic type, a vector force type, etc.
These touch panels have various disadvantages and are not suited to certain applications.
The resistive-film touch panel senses touch when an electrically conductive transparent film interposed between two substrates is touched. The resistive-film touch panel may be more easily implemented than any other touch panel and has high performance. However, the resistive-film touch panel has low mechanical strength and weather resistance.
The optical-sensor touch panel is operated to detect a position when an optical path is blocked between an optical output element and an optical input element. However, the optical-sensor touch panel is affected by light.
The ultrasonic touch panel is operated to detect a position when a sound wave path is blocked between a sound-wave generating element and a sound-wave receiving element. However, the ultrasonic touch panel is vulnerable to noise.
The electromagnetic touch panel is based on Faraday's law of induction and is operated to determine coordinates by calculating the amount of current flowing in a coil at positions on the coordinates. A special pen is required to apply alternating current to operate the electromagnetic touch panel.
The capacitive touch panel senses touch based upon changes in capacitance occurring when a finger contacts a sensor electrode. The conductive touch panel is vulnerable to noise. However, the touch panel of this type may have high weather resistance and high mechanical strength if an appropriate protective layer is employed.
The capacitive touch panel is categorized into an analog type and a digital type. FIG. 1 shows a conventional analog electrode structure and FIG. 2 shows a conventional digital electrode structure.
Referring to FIGS. 1 and 2, the analog electrode structure employs a sheet type sensor electrode 10 and does not require a pattern of electrodes within a sensing area, and the digital electrode structure employs a pattern of sensor electrodes 20 within a sensing area. Each of the analog electrode structure and the digital electrode structure includes a protective layer (not shown) having a certain dielectric constant to protect the sensor electrode 10 or 20 from the external environment.
The electrode structure may detect a position by calculating the amount of current flowing through a finger, which can be a conductor of a ground component, when an alternating current signal is applied to the sensor electrode 10 or 20. Alternatively, the electrode structure may detect a position by detecting capacitance changed by a finger when an alternating current signal is applied to one of a pair of electrodes.
In the analog electrode structure of FIG. 1, the coordinates of a position are determined based upon the amount of current, which is measured at four corners, flowing through a finger. Since there are four lead electrodes 12 which transmit electrical signals to the sensor electrode 10 of the touch panel, the width of an area where the lead electrodes 12 are placed is not significantly increased although the size of a screen of the touch panel is increased. Hence, the analog electrode structure permits reduction in weight and thickness of a large scale panel lighter and thinner. However, since the amount of current is calculated as a relative value, a circuit controller is complex and is vulnerable to external noise.
Further, if there are multiple contact points, it is difficult to realize a multi-touch function, since the contact point is recognized as a middle point of the contact points.
Although the digital electrode structure is also vulnerable to external noise, the digital electrode structure can avoid a change of contact point due to a change in initial value resulting from change in temperature, humidity, resistance, etc.
FIGS. 3 and 4 show the digital electrode structure in more detail. That is, FIGS. 3 and 4 show a conventional digital electrode structure arranged on the X-axis and the Y-axis, respectively.
Referring to FIGS. 3 and 4, position sensor electrodes 31, 41 and signal-transmitting bridge electrodes 32, 42 are formed on a substrate 30.
FIG. 5 is a cross-sectional view taken along line B-B′ of FIG. 2.
Referring to FIG. 5, the sensor electrodes 20 are discontinuously disposed on upper and lower sides of the substrate 21 and are covered with a protective layer 30 to protect the sensor electrodes from external force.
Unlike the analog electrode structure, when an alternating current signal is applied to X-axis and Y-axis grids where the sensor electrodes 20 are connected to one another, the digital electrode structure measures variation in current flowing through a touch point of a finger on the X-axis and the Y-axis and recognizes a position of a point exhibiting the largest current variation as a contact position of the finger.
The digital electrode structure uses an alternating current signal flowing through parasitic capacitors C1, C2, C3 (see FIG. 5) to calculate coordinates of a touch point. In order to obtain accurate coordinates of the touch position, a predetermined number or more of position sensor electrodes 20 must be disposed per unit area. Next, a mechanism to recognize accurate coordinates of a touch point will be described in detail.
FIG. 6 is a graph illustrating capacitive coupling characteristics of a small-sized digital electrode structure according to a related art.
Referring to FIG. 6, accurate coordinates of a touch point may be obtained since there is a sufficient density of position sensor electrodes which communicate signals with a finger. In order to use signals obtained from discontinuous sensor electrodes when calculating the touch coordinates, the signals from each of the sensor electrodes may be represented by normalized Gaussian curves. In this case, a point with the greatest value obtained using a Gaussian function approximated in a region where a signal level is not present between the electrodes may be set as a position coordinate of a point touched by a finger.
Unlike a small-sized digital electrode structure, however, a large scale digital electrode structure may not use the Gaussian function since at least two electrode signals cannot be obtained, as shown in FIG. 7. FIG. 7 is a graph illustrating capacitive coupling characteristics of a conventional large scale digital electrode structure.
Even in the large scale digital electrode structure, an inactive area for signal-transmitting wire electrodes is not significantly increased in width as compared with the small-sized digital electrode structure. In general, an inactive area of a touch panel is designed to be almost the same as an electrode design margin, i.e., a non-viewing area, of a liquid crystal display. The non-viewing area of the liquid crystal display is designed to have a width of 2-3 mm, regardless of the size of the liquid crystal display.
Since the size or number of signal-transmitting wire electrodes is affected by the width of the inactive area, the number of position sensor electrodes provided in a display area cannot be increased compared with the small-sized digital electrode structure.
Hence, since a distance between the sensor electrodes is widened, an area of each sensor electrode is also increased. In this case, increased parasitic capacitance between a finger and the sensor electrode leads to increased coupling. However, the number of sensor electrodes which will be capacitatively coupled with the finger cannot be increased.
If the area of the sensor electrode which will be capacitatively coupled with a finger is increased to a predetermined value or more, it is difficult to sense minute movement of the finger. Thus, the size of the sensor electrode must be determined to allow at least two X-axis and Y-axis sensor electrodes to be present within a contact area when a touch panel is touched using a finger. That is, since the large scale digital electrode structure cannot accurately detect touch coordinates, the size of display devices to which the large scale digital electrode structure can be applied is limited.